Biomass can get a bad press. The very idea of burning trees to
produce power can send some environmentalists into a tizzy. Add into the
mix the fact that a tree felled in South America is shipped across the
world to be burned, and that industry says biomass provides a renewable,
low-carbon source of power, and the result is a frenzy of protest.

But the reality is that the age of biomass will soon be upon us.
The UK's largest coal-fired power plant is already being converted
to run on the stuff. Environmental legislation is calling a day on older
coal-fired power stations, and other renewable technologies are not yet
producing power on the scale that we need to replace them, so biomass
could help to plug the gap.

The country needs to get carbon emissions down quickly and
converting coal plants to run on biomass is one way to do it, says Dr
Patricia Thornley, senior research fellow at the Tyndall Centre for
Climate Change Research. "We have a big fleet of coal-fired power
stations. If we can maximise the use of biomass in those, that goes a
long way to getting carbon reductions in the near term," she says.

Under the Climate Act 2008, the UK is legally bound to reduce
greenhouse gas emissions by 90%, based on 1990 levels, by 2050. A range
of measures will be needed to meet the target--if indeed it is
possible--including the conversion of power plants that were originally
designed to burn coal to take biomass. But the modification of
decades-old plants brings a unique set of challenges.

Many plants up and down the country have been burning biomass
alongside coal for years. Indeed, all 16 of the major plants now do
this. With this "co-firing", 10-20% of the feedstock to a coal
power station is made up of wood pellets.

Different power stations have adapted to co-firing in different
ways, but essentially it involves combining biomass with coal. The
biomass is first pelletised, then ground into a fine powder alongside
coal in the existing mills at the plants. It can be burnt in the same
way as the coal without detrimental effects on boiler tubes and internal
combustion equipment.

But these modifications will take co-firing only so far. Increasing
the proportion of biomass above 20% requires more radical engineering
changes to deal with its different characteristics. Biomass has higher
volatiles, for example, so the combustion unit needs to be reconfigured
to cope with this.

Meanwhile, critics say that the partial conversion of existing coal
plants to use biomass is giving a lifeline to the ailing coal industry,
which collects subsidies from the government in exchange for using
renewable fuel. "In some cases, the coal-fired plants wouldn't
continue to be economically viable were they not able to get the
renewable obligation certificates for the biomass," says Thornley.
However, there is not enough capacity in the generating system to
survive without these plants. "We need those plants to be
continuing, and if we can get them to continue with a lower carbon
intensity, that is a good idea," she adds.

Another biomass option has been purpose-built plants. Dedicated
plants can take a wider range of biomass, including forest residues, but
they cost money to build. There can also be difficulties in getting
planning consent and the permits that are needed to run a power station.
Existing facilities already have these in place, as well as the
transport links required to bring in feedstock.

Given all these factors, it is cheaper to convert an established
coal plant to run on biomass than to build an equivalent new one,
Thornley believes.

Some of the big energy companies agree. At Drax, the UK's
largest power station, in Selby, North Yorkshire, an ambitious project
is under way to convert three of the six generating units to run solely
on biomass.

Conversion of the first unit is well advanced, and this switched to
operate on pure biomass in early April. Engineers are still tweaking the
mechanisms within the generating unit that enable it to run on biomass,
while work continues to get the new fuel storage and handling facilities
up to scale to serve half of the plant. The second generating unit is
due to switch to biomass in 2014, and the third in the following two
years.

"If Drax has a future, it has to move away from coal,"
says Peter Emery, production director at the power station. "For
us, this project is a way to innovate away from economic
extinction."

The advent of the carbon tax increases the pressure, he adds.
"The new carbon tax came in from 1 April, and that is going to
escalate over the next 10-15 years. Our view is that this will quickly
make coal a marginal fuel in the UK energy mix."

In 2005, the company developed a strategy that would decarbonise
power generation. Top of the list was improving the efficiency of the
plant to reduce carbon emissions overall. Work began immediately on a
100 million [pounds sterling] project with Siemens to upgrade the high-
and low-pressure turbines at the station. This project was completed
last year.

At the same time, longer-term options to reduce emissions through
carbon capture and storage were looked at, and work continued to replace
coal with a low-carbon fuel.

Drax had already been working with small quantities of biomass. In
2003-4, it was firing 50,000 tonnes a year alongside coal. By 2006-7,
this figure had ramped up to 250,000 tonnes. But it then became clear
that, to make a step change in co-firing, the power station would need
to invest in bespoke facilities. By engineering and commissioning a new
direct-injection co-firing mechanism, the company was able to boost the
biomass contribution to 1.5-2 million tonnes a year--around 15% biomass
on each of the station's six units.

The plan was to develop the co-firing capacity of each unit further
to 50%, but a sudden change in government policy in 2011 turned the
project on its head. The coalition, which had supported biomass
co-firing when it first came to power, decided to back full conversion
of existing stations to biomass. At Drax, the news caused an
"engineering headache", says Emery, because work on the
project to boost co-firing had already started.

Plans for the facility to receive fuel by rail and develop storage
units did not need to be changed too radically--switching to 100%
biomass on three of six generating units requires the same volume of
biomass as converting all units to run on 50%. But the engineering of a
new fuel distribution system and conversion process had to be
fast-tracked, says Emery.

The biggest challenge was upping the quantity of biomass to be
burned to 100% of fuel, he says. The calorific value of the material is
two-thirds that of coal, so more must be burned to give the same output.

"The concept inside the boiler has changed dramatically, and
we had limited time to prove this last year before we designed the
project, so there is a lot of optimisation going on," he says.

In the unit that is up and running, the flame detection system is
still being tweaked. "That is an important safety feature. You do
not want to be introducing fuel into a furnace where there is incomplete
combustion, as this leads to fires and explosions," he says. The
system is being

adapted to detect a biomass flame, which differs from a coal flame.

A set of biomass burners is in place, but the technology is
relatively new and engineers are still testing several types of burner
for full combustion and a stable flame. Work is ongoing to optimise the
air flow through the burners, and a boosted over fire air system has
been designed to help control nitrogen oxide emissions.

Managing how biomass burns is proving tricky. "Biomass tends
to burn more slowly than coal, so we get higher temperatures at the back
of the furnace," says Emery. "Secondly, biomass can vary --it
has elements such as alkaline metals, chlorine and silica, and they can
cause fouling." The performance of a fouled furnace falls quickly,
so Drax is working to manage fuel quality and include additives that can
mitigate fouling and corrosion.

Outside the furnace, there are other challenges. More biomass is
needed to produce each unit of energy compared with coal. "It is
two-thirds of the bulk density--if you do the maths, you are handling
broadly double the volume," he says. The additional material has to
be handled and stored.

Unlike coal, biomass swells to three times its size when wet and
cannot then be burned, so covered storage has had to be designed and
built. By contrast, coal at Drax is stored in open stacks. The biomass
tends to be very dry and produces a lot of dust, which can cause
problems. Biomass will self-heat, just as a compost heap does, and can
ignite. Even relatively small piles of biomass dust can self-heat,
leading to an explosion risk. So it needs to be monitored and managed in
a way that is not required with coal. "You do have similar issues
with coal dust, but far less because coal is quite wet when you
introduce it to the mills. It's harder to ignite in smaller
quantities," says Emery.

The dust is hazardous and, unless the biomass is completely
enclosed, will pose a health risk to operators. "From an
operational perspective, you have to treat biomass with even more
respect than coal," he says.

As part of the conversion, the rail unloading and storage system
has been redesigned to minimise dust production. The conveyors are fully
enclosed, and the system has new transfer points in an attempt to
minimise the physical impact on the biomass every time it moves from
conveyor to conveyor. The storage domes that will house the biomass are
designed to minimise and contain any dust. Where possible, low-heat
lighting is used to remove potential sources of ignition. From the
storage domes, the biomass travels in a fully enclosed system to day
silos, where it is transferred to a pneumatic system that takes it by
pipe to the generating unit. The biomass is milled and injected by
blowers into the unit. At the moment, the existing direct-injection
facilities are in use, but a new system is being commissioned.
"That is a massive change from how we treat coal," says Emery.
In coal units, the fuel passes through a series of bunkers before it is
milled and blown into the burners.

Across the project, the business has taken "some leaps of
faith" to deploy the new technology, he says. The output of the
units that run on pure biomass will be 585MW, around 10% less than the
units that run on coal, but work will continue to improve that. The unit
operating on pure biomass has an 80% reduction in carbon emissions
compared with those that take coal, he says.

Converting the station will not come cheap. Drax is spending 300
million [pounds sterling] on-site alone. Up to 1,000 contractors will be
on the site for almost 18 months to achieve the transformation. The same
sum again will be spent to develop pellet plants in Louisiana and
Mississippi, and a port facility in Baton Rouge, so the biomass can be
shipped to Drax. The business has also secured biomass from elsewhere in
North America. the Baltics, Europe and Africa But more is still needed.

Sourcing enough biomass for the project has been tough, says Emery.
"It's a little bit 'chicken-and-egg', because people
don't want to commit to pellet plants f they are not convinced you
can stand behind a longer-term contract, and we can't justify
converting the station until we nave got a secure supply of
pellets."

The biomass supply industry is still in its infancy, but looks set
for growth. Up and down the country, many coal-fired plants are running
on limited time and are set to close under the large combustion plant
directive (LCPD)--legislation designed to control acidification,
ground-level ozone anc particle pollution across Europe by reducing
nitrogen and sulphur oxides and ash emissions from large power plants.
Converting the stations to run on 100% biomass may prove an appealing
option. RWE has converted its plant in Tilbury in Essex to run on pure
biomass (see box, above). The plant had been scheduled to close under
the LCPD, but RWE is now looking to extend its life for a further 10-12
years.

So far, the plans of other operators are less advanced than those
of Drax and RWE. E.On is considering converting its Ironbridge plant to
run on pure biomass, as is Eggborough Power, which runs a 2,000MW plant
in North Yorkshire. Both companies declined to comment on progress for
this article.

Benefits add up despite thousands of shipping miles involved

Exactly how green are biomass power plants, given that their fuel
comes from forests that are vital to the earth's carbon cycles?
After trees are chopped down and processed, the biomass is then often
shipped halfway around the world for burning. But if forests are
properly managed, biomass is a credible source of renewable energy, says
Dr Patricia Thornley, senior research fellow at the Tyndall Centre for
Climate Change Research.

She has calculated the carbon footprint for several biomass systems
and says the carbon reductions for electricity plants are in the order
of 80% lower than the UK grid average. The carbon reduction benefits are
less if the biomass needs substantial processing, such as pellatising.
Pelletising brings the carbon reduction benefits to 60-70% less than the
UK grid average, she says.

How far the biomass travels to reach the plant, and how it is
transported, are other factors in determining the carbon footprint. But
the effect of transporting the fuel thousands of miles is not as large
as people might think, she says. "Shipping is really efficient and
doesn't actually have that big an impact on the carbon
footprint."

One of the biggest issues to come to light recently is how changing
land use to grow biomass crops can alter the dynamics of carbon exchange
in the atmosphere. The impact of land use change needs to be accounted
for in calculations, and this is not straightforward. "Taking into
account the carbon all the way upstream is incredibly difficult,"
she says.

"The government is trying to be sure that the biomass that we
are using is having carbon benefits. It is putting effort into getting
the certification in place and getting it right," she adds.

Research is ongoing into how the carbon balance of ecosystems might
be affected by changes in forestry patterns and land use, but Thornley
says she is happy with the biomass that is coming into the UK. "The
vast majority of it is delivering carbon reduction benefits compared
with what we have on the national grid."

Full conversion cuts emissions at Tilbury B

RWE converted its Tilbury B power station in Essex to run on 100%
biomase fuel in 2011, saying that this option was many times cheaper,
and leas time-consuming, than building anew. Previously, the plant had
been co-firing coal and biomass for seven years. The switch to pure
biomass has reduced greenhouse gas by more than 70% compared with coal,
as well as reducing nitrogen and sulphur oxide emissions and ash, says
the company.

The power station uses wood pellets that are mostly sourced from
the south-eastern US or Canada, with some coming from RWE's own
plant in Georgia, and that are shipped to the UK.

The engineering behind the conversion took just four months, says
RWE. Switching to biomass required modifications to the mills and
classifiers to accommodate the fuel, because the pellets have a
different diameter from coal. Engineers modified the burners to optimise
combustion and improve the stability of the flame.

The main challenges were the handling, storage, maintenance and
transfer of the biomass, says the company. To reduce escaping dust when
the wood pellets are being unloaded from ships, it has installed two
dust-suppressing vacuum unloaders.

RWE has planning permission to convert Lynemouth power station in
Northumberland to run on biomass, but has yet to make a decision on when
the conversion will happen.

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